In geology, faults are discontinuities (cracks) in the Earth's crust that are the result of differential motion within the crust. Faults are the source of many earthquakes that are caused by slippage vertically or laterally along the fault. The largest examples are at tectonic plate boundaries, but many small faults are known to exist that are far from active plate boundaries.
The two sides of a fault are called the hangingwall and footwall. By definition, the fault always dips away from the footwall. Faults can be categorized into three groups: normal faults, transform (or strike-slip) faults and reverse (or thrust) faults.
This occurs when the crust is in tension. The hangingwall moves downwards (i.e. towards the centre of the Earth) relative to the footwall. The depressed ground between two parallel normal faults is called a graben. A ridge between two parallel normal faults is called a horst.
Reverse (or thrust) fault
This occurs when the crust is in compression. The hangingwall moves upwards (i.e. away from the centre of the Earth) relative to the footwall.
Schematic illustration of normal and reverse faults. Note that the view is a cross-section through the Earth, such that the up-direction on the page is away from the centre of the Earth.
Strike-slip faults
The fault surface is vertical and the footwall moves either left or right (with respect to the plane perpendicular to the fault and to the Earth's surface). Strike-slip faults with left-lateral motion are also known as sinistral faults Those with right-lateral motion are also known as dextral faults.
Schematic illustration of the two strike-slip fault types. The view is of the Earth's surface as from space.
In detail, the fault is a complex zone of crushed and broken rock from a few hundred feet to a mile wide.
The fault zone is marked by distinctive landforms that include long straight escarpments, narrow ridges, and small undrained ponds formed by the settling of small blocks within the zone.
During the 1906 earthquake in the San Francisco region, roads, fences, and rows of trees and bushes that crossed the fault were offset several yards, and the road across the head of Tomales Bay was offset almost 21 feet, the maximum offset recorded.
Here the San Andreas fault interacts with other faults (most notably the San Jacinto fault zone and the Pinto Mountain fault) and thereby becomes somewhat fractured, over the distance extending from just north of San Bernardino to just north of Indio, some 110 kilometers (70 miles).
Fault rupture mechanics are still not well understood, however, and the discontinuity could prove to have little effect on tempering a major earthquake on this southern stretch of the San Andreas fault zone.
It should be noted that due to the complexity of this area, many researchers have used different nomenclature for the local faults, and placed the dividing lines between certain named fault segments in varying places.
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